Study: The Glycemic Index is Unreliable

glycemic index
Science & Medicine

When it’s times to eat, people with diabetes look to the glycemic index (GI) to tell them how a certain food will impact their blood glucose. However, new research suggests the GI may not be as reliable as previously thought.

What is the Glycemic Index?

The glycemic index ranks carbohydrate-containing foods based on how they raise blood glucose, or blood sugar. A food with a high GI raises blood glucose more than a food with a medium or low GI. [1]

If you have diabetes and you know you’re going to be eating something with a high GI, you can combine it with low-GI foods to help balance the meal.

Examples of low-GI foods include:

  • Rolled or steel-cut oats
  • Sweet potatoes
  • Most fruits, and non-starchy vegetables and carrots

Some medium-GI foods would be:

  • Whole wheat bread
  • Quick oats
  • Brown Rice

High-GI foods include:

  • White bread
  • Pretzels
  • Pineapple

The Latest Study

For the latest study, which is published in The American Journal of Clinical Nutrition, researchers looked at blood sugar responses in 63 healthy adults after eating the same amount of white bread – a high-GI food – 3 different times over 12 weeks. [2]

The team found that glycemic index values varied an average of 20% among individuals, and 25% between different study participants.

In plain English: each participant’s blood sugar levels varied by 20%, even though everyone was eating the same food.

This means that specific foods like do not have consistent glycemic indexes, so the GI may not be as useful, or as reliable, as doctors and diabetics have long believed it to be. [3]

Based on this new information, the researchers believe there is no point in assigning a GI to foods.

Lead study author Nirupa Matthan, a scientist at the U.S. Department of Agriculture Human Nutrition Research Center at Tufts University in Boston, says:

“Glycemic index values appear to be an unreliable indicator even under highly standardized conditions, and are unlikely to be useful in guiding food choices.

If someone eats the same amount of the same food three times, their blood glucose response should be similar each time, but that was not observed in our study. A food that is low glycemic index for you one time you eat it could be high the next time, and it may have no impact on blood sugar for me.” [2]

She goes on:

“If your doctor told you your LDL [‘bad’] cholesterol value could vary by 20 percent, it would be the difference between being normal or at high risk for heart disease. I don’t think many people would find that acceptable.”

The researchers didn’t provide a better alternative to the glycemic index. But since the GI does indicate which foods spike your blood sugar faster than others, it shouldn’t be completely dismissed. As a diabetic, I can tell you I will continue to use the GI because it’s the best guide I have at the moment.

Just make sure you monitor your blood sugar, and keep in mind that the glycemic index isn’t written in stone.


[1] American Diabetes Association

[2] HealthDay

[3] ABC News